Differential resonant (i.e., inductor/capacitor (LC)-based) oscillators are increasingly being used to perform low-jitter frequency synthesis in integrated circuit (IC) systems. This trend has been made possible as a result of the relatively recent feasibility of implementing inductors monolithically with good quality factor, Q, using interconnect wiring metal layers. In qualitative terms, Q of a resonant system is the ratio of the total energy in a system to the energy lost per cycle. FIG. 1 illustrates a block diagram of a resonant oscillator circuit 1 having a pair of resonant LC tanks 2 and 3 that consist of ideally identical inductors 4 and 5 and variable capacitors 6 and 7. Each resonant tank oscillates differentially with respect to the other tank at a frequency, f=½π(LC)0.5, where L is the value of the inductance of the tank inductor and C is the value of the capacitance of the tank variable capacitor. Cross-coupled gain transistors 8 and 9 periodically replenish energy into the tanks 2 and 3 to sustain oscillations that would otherwise decay and disappear due to parasitic resistive losses in the inductors and capacitors. Tunable output frequencies are typically generated by modulating the capacitance of the variable capacitors 6 and 7 using some control voltage, Vcontrol.
FIG. 2 illustrates a perspective view of a known planar spiral inductor 11 formed in an IC using a single layer of interconnect metal to form, for example, three turns 12, 13 and 14. The first turn 12 of the inductor 11 starts at end 15 and the third turn 14 ends at end 16, which is interconnected to a feed 17 by vias 18 and 19 and underpass element 20. Such inductors can be built to exhibit a relatively good quality Q due to the fact that the physical separation between the highest interconnect level, which is typically where the inductor is formed, and the semiconductor substrate below ensures that minimal energy will be dissipated as a result of eddy currents being magnetically induced in the substrate. However, because of weak mutual magnetic coupling between inductor windings, these inductors typically need to be extremely large in order to achieve a target self-inductance, and thus consume a large area in the IC, making implementation rather expensive.
It is known to create a differential resonant oscillator in an IC by using a pair of the planar spiral inductors shown in FIG. 2 to achieve a circuit design of the type shown in FIG. 1. In differential resonant oscillators, the mutual inductive coupling between the two planar spiral inductors is usually tailored to provide very strong magnetic coupling between the two inductors. Strong magnetic coupling between the inductors mitigates problems that may occur due to asymmetries and mismatches between the left and right resonant tanks that occur during IC manufacturing. Strong coupling can also prevent undesirable nonlinear effects that may cause the left tank to behave in a non-differential fashion from the right tank. Without strong coupling, the two resonant tanks can oscillate independent of each other in a non-differential fashion.
The coupling of the two tanks through the negative impedance generator (i.e., the cross-coupled gain transistors 8 and 9) is typically insufficient to eliminate the effects caused by tank asymmetries. One such effect is the left tank oscillating with a different voltage amplitude and non-180° phase alignment from the right tank due to large voltage amplitude oscillations about typically very nonlinear capacitance-versus-control-voltage characteristics of the variable tuning capacitors 6 and 7. Such instabilities can produce undesirable oscillator output jitter.
One known practical way of tightly coupling the two tanks is implementing strong magnetic coupling of the spiral planar inductors of a differential resonant oscillator through cross-coupling of the inductors. FIG. 3 illustrates a perspective view of a cross-coupled pair 21 of planar spiral inductors 22 and 23. For ease of illustration, each inductor is shown as having a single turn. Inductor 23 is cross-coupled with inductor 22 by vias 24 and 25 and cross-coupling element 26.
Although tight mutual coupling can be achieved with the cross-coupled planar inductor pair 21 shown in FIG. 3, the inductor pair 21 consumes a relatively large amount of area on the die. Since die cost is commensurate with area, area can be a significant impediment to practical implementation of certain circuit architectures and applications. Moreover, the orientation of the turns of the inductors 22 and 23 is such that there is very strong negative magnetic coupling. When the differential nature of the left and right tank oscillations is taken into account, the polarity of this net negative coupling will be flipped to yield net additive magnetic linkage between the two inductors 22 and 23 and the substrate. This linkage can lead to energy being dissipated as a result of eddy currents being magnetically induced in the substrate and lower the inductor Q.
In addition, the resulting orientation of the two planar spiral inductors 22 and 23 creates another key drawback. When the right-hand rule is applied to determine the orientation of the magnetic flux lines, it becomes apparent that the magnetic fields from the differentially driven inductors are additive as they penetrate through the substrate and surrounding vicinity of the inductors. This will induce noise through eddy current generation, which can limit the number of resonant oscillators that can be monolithically integrated in a single IC die.
A need exists for an inductor pair formed on an IC that has strong mutual magnetic coupling between the inductors, that has low energy loss due to eddy currents being generated in the IC substrate, and that consumes a small amount of area on the IC die.